In the case of high-performance vessels, such as those intended for storing fluids under pressure, in particular for space applications, or for storing cryogenic fluids under pressure, the metal liners are thin in comparison to the vessel's volume.
‘High-performance vessels’ means vessels optimized in terms of mass, such as those used in transportation industries in general, space transportation in particular.
High-performance composite vessels intended for storing pressurized fluids are generally designed by separating the functions of hermeticity and mechanical resilience to pressure.
Containment of the fluid, hermeticity and/or protection of the wall made of composite material with regard to the fluid is provided by a shell made of metal or polymer, generally thin, called the “liner”; and resistance to the pressure of the fluid contained in the vessel is provided by a winding of composite fibers coated with resin.
The composite fibers (fibers and resin) are deposited on the liner by filament winding.
This technology is known and Lockheed Martin's patent U.S. Pat. No. 6,401,963 can be cited as an example.
The shell is thin since it has no mechanical function and the goal sought is to minimize the vessel's mass.
In addition to its hermeticity function, the liner also has the function of withstanding the forces induced by depositing the carbon fibers, during which the liner functions as a mandrel for the winding, and provides the reference surface for the composite structure deposited.
This secondary winding support function performed by the liner gives rise to a need for the mechanical resilience and/or stiffness of the liner.
This requires the liner to be sufficiently thick, depending on the properties of the material and geometry of the part.
The winding forces appear as a local compression on the liner of approximately 0.3 MPa and consequently appear as increased mass of the finished vessel.
This can be a particular problem for large-sized vessels, several meters in size, where the liner's mass becomes significant, or when the material of the liner is ductile and/or has a low elastic limit and/or low mechanical resilience; this is the case, for example, for a liner made of high-purity aluminum, or in general when, to reduce the liner's mass significantly, thicknesses of less than 1 mm are reached for vessels with dimensions in meters.
The liner's resistance is also an important parameter in the case where the fibers are placed by contact winding or fiber placement.
In effect, these methods apply a force directly onto the support at the point where the fibers are deposited, whereas in winding this force results from the tension to which the thread is subjected.
In the case where the liner is not able to withstand the thread deposition forces, or when a hermetic liner is not needed, a mandrel is used.
There are many mandrel technologies, and metal mandrels and mandrels made of a soluble material (e.g. sand plus polyvinyl alcohol soluble in water) can be cited.
A general problem is removing the mandrel after winding and producing the composite material because, in general, the mandrel is only accessible through the vessel's future filling port.